Well the Austrians are right in that the traditional assumptions of rationality (and therefore fairness) don't make much sense.

Although I think the behavioral economists are doing a decent job of backing up these type of assertions: the point that "fairness" is subjective.

Can someone engage in a trade they don't believe is fair? Yes of course.

But if both parties say a trade is fair, who is an outside observer (with all the market data in the world) to tell them otherwise?